Couple of things. Like I said on the other thread, that's very good strength for age/weight. IMO you could tweak your technique a little to get more out of sumo as follows.
1 - this'll sound odd, but be slower off the floor. I've found sumo works better if you focus on levering the bar up rather than pulling as you would for conventional deadlift. Once the bar is moving (around mid-shin or a bit higher), THEN accelerate and slam your knees shut.
2 - pull your shoulders further back behind the bar when you set up, and do everything you can to keep them there as you raise your hips to break the bar off the floor. This will most probably stop your hips shooting up so fast and force your to use your glutes/hams hips to move the bar which will in turn help with levering the bar up gradually.
I know it sounds odd for a deadlift to advise being slow off the floor but with sumo my understanding and experience is that it is so much harder to get back into position if you come out of it early on that the focus must be on locking into the right position and staying there. Basically, if your shoulders come forward in the early stages of the pull your leverages go to hell and you essentially end up with a conventional DL with a horribly wide stance which puts your leverages out of whack. If you lock your shoulders back behind the bar as you start the lift with your hips/glutes/hams, you keep a good arch in your back and stay in the best position. It'll make you a bit slower off the floor but the whole lift easier.
If its any help, to set up for sumo I imagine a right angle triangle: the base is my thighs, parallel to the ground. There is a line perpecndicular to my thighs going straight up through the bar, behind which I pull my shoulders. My back makes the third side. To start the pull, all I do is open the angle between the base and my back by raising my hips and pushing through the sides of my feet to pull the floor apart, keeping my shoulders behind the line.